The half-life of a radioactive isotope is three hours. If the initial mass of the isotope were $256…

The half-life of a radioactive isotope is three hours. If the initial mass of the isotope were $256 \mathrm{~g}$, the mass of it remaining undecayed after 18 hours would be
  1. $8.0 \mathrm{~g}$
  2. $12.0 \mathrm{~g}$
  3. $16.0 \mathrm{~g}$
  4. $4.0 \mathrm{~g}$

Solution

$\mathrm{t}_{1 / 2}=3 \mathrm{hrs}$. Initial mass $\left(\mathrm{C}_0\right)=256 \mathrm{~g}$ $\therefore \mathrm{C}_{\mathrm{n}}=\frac{\mathrm{C}_0}{2^{\mathrm{n}}}=\frac{256}{(2)^6}=\frac{256}{64}=4 \mathrm{~g}$

Asked in: JEE Main 2003

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